This invention relates to a transducer utilized to measure particles or cells suspended in a fluid.
A number of different techniques have been developed for measuring or counting particles in a fluid. An early technique was developed wherein the particle containing fluid was passed through an aperture having a relatively small diameter. An electric current was conducted through the aperture and the impedance variations of the particle containing fluid in the aperture was measured. The presence of a particle altered the impedance characteristics of the current conducting path through the aperture and therefore gave an indication of the size of a particle. Under certain circumstances, if the particle size was known, other characteristics of the particle could be deduced. A detailed description of this measuring technique is presented in Coulter U.S. Pat. No. 2,656,508 and Coulter et al. U.S. Pat. No. 2,869,078.
After this technique was developed, it was recognized that as the particles in the fluid suspension pass through the aperture, the particles flow or whirl back into the immediate vicinity of the downstream end of the aperture to cause another variation in the impedance of the current path. This impedance variation is caused by the particles entering an area of relatively high current density proximate the aperture. This gave rise to spurious or erroneous readings which were unacceptable. Various techniques have been developed to overcome this problem none of which are completely satisfactory. One such technique is illustrated in Hogg U.S. Pat. No. 3,299,354 wherein an aperture tube is provided which has two chambers. The first chamber has an aperture for permitting the passage of the particle containing fluid therethrough. The second chamber has an open end or orifice terminating at a point very closely spaced from and in alignment with the downstream end of the aperture in the first chamber. The purpose of this structure is to separate the electrical and mechanical effects of the particles passing through the aperture. This development has the drawback, however, of generating eddy currents of fluid in the aperture tube at the downstream end of the first chamber and these eddy currents swirled into the orifice of the second chamber. This caused the particles to swirl back into the high current density area proximate the aperture to thereby generate error in the testing of the particles in the fluid suspension.
In order to overcome this, a system was developed as disclosed in Hogg U.S. Pat. No. Re. 28,558 wherein substantially the same tubular structure was utilized as in the earlier Hogg '354 patent. However, a pump was provided to generate a continuous flow of the particle containing fluid in order to draw the particle containing fluid through the aperture and through the orifice of the second chamber so that the particles were continuously transported away from the high current density zone or region proximate the aperture. As acknowledged in a later patent, i.e., Hogg et al. U.S. Pat. No. 3,902,115, the structure disclosed in U.S. Pat. No. Re. 28,558 was relatively fragile and cumbersome and difficult to manufacture with the tolerances required. In addition, the flow velocity required in order to sweep out particles after they passed through the aperture had to be extremely high and, therefore, in order for the aperture to work, either a very minute restriction area downstream of the aperture between the aperture and the second chamber was required or exceedingly large quantities of fluid had to be pumped through the aperture tube.
To overcome this problem, and others associated with this structure, Hogg et al. developed, as illustrated in U.S. Pat. No. 3,902,115, an aperture tube structure wherein an electrolyte is pumped through one chamber of an aperture tube and into a second chamber either through an orifice or past a restriction which is positioned in very close proximity to the aperture. The orifice or restriction serves as a means for generating a high sweep velocity to cause all particles proximate the aperture to be drawn away from the high current density or spurious signal zone and into the second chamber. As in the case of the '115 patent, exceedingly high sweep velocities are required in order to make the tube operate as intended and, therefore, structurally a very minute restriction downstream of the aperture is required. Such a restriction is not only difficult to form structurally because of the tolerances required, but also generates a high series electrical resistance to the current flow through the aperture area. Further, this arrangement requires a relatively large flow of electrolyte for clearing the particles from the spurious signal zone near the aperture.
Another technique, as illustrated in simplified form in FIG. 1, uses the basic apparatus disclosed in Coulter et al. U.S. Pat. No. 2,869,078. As illustrated, a flask 11 contains an electrolyte which is used to wash or remove particles from the spurious signal zone proximate the aperture 13 of an aperture tube 15. A stopcock 17 is partially opened and a vacuum which is established in the aperture tube 15 via control valve 18 draws the electrolyte from flask 11 through conduit 19, the stopcock 17, and through tube 21 into the aperture tube 15. The particle containing fluid is located in a container 23 and this fluid is drawn into the aperture tube 15 through aperture 13. The electrolyte from flask 11 serves to dilute and wash away the particles passing through the aperture 13 and into a waste flask 25. The electrolyte has the effect of diluting the area behind the aperture 13 thereby reducing the probability of cells or particles reentering the spurious signal zone proximate the aperture. This technique measurably reduces the number of spurious signals generated but requires a substantial amount of electrolyte and, in addition, does not completely eliminate spurious signals.
It, therefore, is an important objective of the present invention to provide a structurally simplified transducer for measuring particles suspended in a fluid medium wherein error due to the recirculation of the particles in a region proximate an aperture in an aperture tube is minimized.